1. Field of the Invention
The present invention is directed to technology and methods for scanning and focusing an ultrasound beam in a free direction within a region of 3D space. The technology has particular applications within 3D medical ultrasound imaging, but also has applications in other areas of 3D ultrasound imaging.
2. Description of the Related Art
Three-dimensional (3D) ultrasound imaging is obtained by scanning a pulsed ultrasound beam in two side-wards directions to the beam axis. Time of flight conversion gives the image resolution along the beam direction (range), while image resolution transverse to the beam direction is obtained by the side-wards scanning of the focused beam.
With 3D imaging one can collect volume ultrasound data from the whole object so that one through computer processing can visualize any cross section of the object. This enables selection of the best 2D image planes for the diagnosis, for example showing a sagittal plane of a fetus where the location of the fetus in relation to the probe complicates direct 2D imaging of the sagittal plane.
Such 3D imaging is of particular interest when imaging from an endoluminal channel with limited ability to move the probe in relation to the object. Typical examples are trans-vaginal imaging of the fetus, trans-rectal imaging of the prostate, imaging during minimal invasive surgery, etc. The method is also interesting for trans-cutaneous imaging to visualize 2D image planes with an angle to the beam directions.
For adequate imaging of the object in freely selected planes or sections in a 3D volume data set, it is important that the ultrasound beam is well focused in all directions around the beam axis, as one wants to observe the object from any direction (perspective), and small objects can be interrogated with a variety of beam directions. In addition, one can want to transmit a wide beam that covers several parallel and well focused receive beams to obtain fast volume data collection with high spatial resolution of the object. In the same way one can want to transmit several beams in different directions in parallel to further reduce the time of volume data collection from the object.
There exists in the market 3D ultrasound probes based on a 2D scanning linear array (switched linear and curve linear as well as linear phased arrays), where the second direction scanning is obtained by mechanical tilting of the 2D scan plane. The scan direction in the 2D scan plane is referred to as the azimuth direction, while the direction normal to the 2D scan plane is referred to as the elevation direction. These 3D probes in the market provide electronic steering of the focus in the azimuth direction to any depth, while the focus in the elevation direction is fixed, which is a drawback with viewing planes at an angle to the beam directions.
For electronic steered focusing of the beam in both the azimuth and elevation directions, one can either use an annular array, or divide the elements of a linear array in the elevation direction for electronic focusing in the elevation direction. The linear array has an advantage over the annular array in that one can rapidly produce large changes in azimuth the beam direction, which is an advantage for Doppler imaging of low blood velocities and velocity and strain rate of moving tissue. The linear array also allows parallel transmit and receive beams in different directions, which can be used to speed up the volume data collection rate to obtain real time 3D imaging, what is commonly referred to as 4D imaging.
For large angle electronic steering of the beam direction in 3D space, the element width must be small (˜λ/2, where λ is the ultrasound wave length) both in the azimuth and the elevation directions. This produces a large number of elements (>˜3000) which complicates the electronic connection to the elements and the electronic steering of the elements, requiring large space for electronics and cables that is difficult to obtain with the narrow channels available for endo-luminal probes.